1. Field of the Invention
The present invention relates to an inkjet printhead, printing apparatus, and printing method. Particularly, the present invention relates to an inkjet printing apparatus which prints by scanning, on a printing medium in a direction perpendicular to the nozzle array direction, a printhead having a plurality of nozzles that are arrayed in a predetermined direction and discharge ink droplets, and a method of driving the printhead of the apparatus.
2. Description of the Related Art
Printers with printing information such as a text or image which a user wants on a printing medium such as paper or a film, have conventionally been known as information output apparatuses in a wordprocessor, personal computer, facsimile machine, and the like. As the printing method of such a printer, various methods are known, including dot impact printing, thermal printing, and inkjet printing. Of these methods, the inkjet printing method is one of the non-impact printing methods, and is advantageous because it can print on a variety of printing media at high speed, and can fix an image even on so-called plain paper without any special processing and provide a high-resolution image at low cost.
With these advantages, inkjet printing apparatuses are rapidly prevailing recently, not only as a printer serving as a peripheral device of a computer, but also as printing apparatuses in a copying machine, facsimile machine, wordprocessor, and the like.
Inkjet types in ink discharge methods widely used at present are classified into a method using an electrothermal transducer (heater), and one using a piezoelectric element. Either method controls discharge of ink droplets using an electrical signal. According to an ink droplet discharge principle using the electrothermal transducer, an electrical signal is supplied to the electrothermal transducer to instantaneously boil ink near the electrothermal transducer (film boiling). An ink droplet is quickly discharged by rapid growth of a bubble generated by phase change of ink at this time. Therefore, this method has advantages of simplifying the structure of the inkjet printhead (to be referred to as a printhead), and easily integrating nozzles.
For high-density printing, the printhead often has a plurality of nozzles for discharging ink, and a plurality of discharge pressure generation elements. In general, the printhead adopts a time-divisional driving method. More specifically, according to this method, the nozzles are divided into a plurality of groups for every predetermined number of nozzles based on physical positions. The nozzles in each group are further divided into driving blocks. The discharge pressure generation elements are time-divisionally driven for each driving block. The divisional driving method is effective for downsizing power supply members such as a head driving power supply, connector, and cable.
Especially in a printhead using heaters, it is necessary to minimize voltage fluctuations and finely adjust the voltage value in order to perform stable discharge in consideration of the characteristics of the heater, ink, and the like. It is not preferable to increase the capacity of the power supply for the discharge pressure generation element. The divisional driving method is effective even for satisfying these requirements concerning the power supply.
An outline of a printing method on a printing medium by an inkjet printing apparatus will be explained.
FIG. 6 is a schematic view showing flying of ink droplets discharged from the printhead.
In a printhead of this example, as shown in a of FIG. 6, a nozzle array 101 has 48 nozzles. The nozzles are divided into six, first to sixth groups, and each group includes eight nozzles. Nozzle numbers 1 to 48 are assigned to the respective nozzles.
As shown in b of FIG. 6, this printhead has eight time-divisional driving blocks such that six nozzles having nozzle numbers 1, 9, 17, 25, 33, and 41 belong to the 0th block, and nozzles having nozzle numbers 2, 10, 18, 26, 34, and 42 belong to the first block. All the nozzles are sequentially driven at eight different timings of a period T in ascending order of the 0th to seventh blocks sequentially in accordance with pulse-like discharge timing signals 301-0 to 301-7 shown in b of FIG. 6. In this way, the plurality of nozzles are time-divisionally driven. In other words, each of the eight nozzles in each group belongs to one of the eight blocks, and upon printing, the nozzles are time-divisionally driven for the respective blocks. That is, nozzles in the same block are concurrently driven. Note that each of numbers “0” to “7” is a value indicating a block (block number).
In the example of FIG. 6, each nozzle discharges an ink droplet 302 to a printing medium 13 in correspondence with a driving signal, as shown in c of FIG. 6. Note that the discharge period T of each nozzle is determined based on position information generated by reading a scale arranged along the carriage moving path by an encoder along with movement of the carriage of the inkjet printing apparatus, and processing the reading signal by the CPU.
FIG. 7 is a block diagram showing the circuit arrangement of the printhead and the arrangement of the print data generator of the inkjet printing apparatus.
A printhead 102 includes 48 heaters 501-1 to 501-48 in correspondence with 48 nozzles. The respective heaters are connected to a heater driving power supply VH, and drivers 502-1 to 502-48 for driving the corresponding heaters. The control input signals of the respective drivers are connected to independent gates 503-1 to 503-48, respectively.
In addition, the input of each gate 503-xx (xx=1 to 48) is connected to a heat enable signal ENB, one of six data lines for respective nozzles, and one of eight driving block signal lines.
Each of the print data signals DATA serially transferred from the printing apparatus is received by a serial-parallel converter 504, and is latched by a latch 505 for each block in synchronism with the input timing of a latch signal LT. This operation is executed eight times, receiving print data signals DATA corresponding to all the 48 heaters of the printhead and supplying print data signals corresponding to sixth nozzles for each input. Since the driving block signal takes eight states of 0 to 7 in order to select one of the eight blocks, the printing apparatus serially transfers 3-bit signals. The 3-bit signal is latched by the latch 505 and converted into eight block selection signals by a decoder 506.
A print data generator 508 in the printing apparatus generates the heat enable signal ENB, the print data signal DATA, a clock signal CLK, and the latch signal LT used to drive the printhead. When a timing adjustment circuit 507 detects a change of a position signal from an encoder 12, the printhead 102 acquires from the print data generator 508, the print data signal DATA to be printed at this position. The print data generator 508 includes a buffer memory, which stores print data transferred from a host (not shown) via an interface (not shown) after converting the print data in the ink discharge order. The print data generator 508 receives even image data read by a scanner or the like.
The timing adjustment circuit 507 decomposes data for the 48 nozzles into data for six nozzles/eight blocks, and transfers the resultant data to a print data converter 509. The print data converter 509 converts data of six pixels (for six nozzles) and block number data into serial signals, and outputs them to the printhead 102.
Each nozzle has nozzle-specific characteristics regarding the ink droplet discharge direction, discharged liquid speed, and the like. The nozzle-specific characteristics may adversely affect a printed image and cause occurrence of a stripe, density unevenness, or the like in the printed image.
FIGS. 8A and 8B are, corresponding to nozzle arrangement shown in a of FIG. 6, views for explaining occurrence of a stripe, density unevenness, or the like in a printed image.
FIG. 8A shows printing positions on the printing medium 13, assuming that the nozzles have a uniform ink discharge characteristic (shift amount=0) and all the nozzles discharge ink at the different timings (DT0, DT1, . . . , DT7) of the period T described with reference to FIG. 6 along with a movement of the carriage. FIG. 8B is a view conceptually showing a shift of the printing position on the printing medium due to the discharge characteristic of each nozzle. For example, the printing position of a nozzle with a block number “0” in the first group shifts rightward in FIG. 8B by 1 when a distance corresponding to ⅛ of the resolution from the reference printing position is defined as unit “1”. For example, a nozzle with a block number “1” in the first group prints at almost the same position as the reference printing position. Further, for example, the printing position of a nozzle with a block number “2” in the first group shifts leftward in FIG. 8B by 2 when a distance corresponding to ⅛ of the resolution from the reference printing position is defined as unit “1”. In this fashion, the printing position shifts for each nozzle due to the nozzle characteristic. As a result, a stripe or density unevenness appears in a printed image.
To reduce the adverse effect on a printed image, multi-pass printing has been used to print an image by scanning a printing area by an inkjet printhead by a plurality of number of times so as to print the same raster using two or more different nozzles (for example, see Japanese Patent Laid-Open No. 2002-103586 (FIG. 1, paragraph [0018]).
However, multi-pass printing has a drawback of decreasing the printing speed because an area printable at once is restricted to, for example, ½ or ⅓ of the nozzle array. To make the stripe or density unevenness less conspicuous, the number of passes needs to be increased.
The inkjet printing apparatus is required to maintain a state in which ink can always be discharged stably. However, as the printing quality becomes higher, it becomes more difficult to suppress manufacturing variations of nozzles and variations of printing elements to a level at which they do not affect the quality of a printed image. Further, this raises the manufacturing cost.